I don't think this will work.

Basically, we can use conditionals where the spell functions one way if the target is friendly [help], but differently if it is hostile [harm].

Unfortunately, the tank is friendly, so any condition I can think of which falls through and heals another party member, would also heal the tank if you happened to be targeting the tank instead of a different friendly.

We cannot use [Focus] as a conditional in a macro.
"/Cast [Help] Renew; [Harm] Mindblast" works fine.
I don't think we can do a macro like this, "/Cast [Focus] Null; [Help] Heal Spell; [Harm] Nuke".

If [Focus] is not an argument, then it won't work.

If your tank is your focus, and you can use focus as an argument, then you could accomplish this via a castsequence and the null command.

/castsequence [Focus] ,Null
/castsequence [Help] Heal Spell
/castsequence [Harm] DPS Spell.

I'm pretty sure the above won't work, but that's about the only way I could see to do it, using the macro logic we have access to.